Preheat oven to 350. Prepare a 8" square dish with cooking spray; set aside. In a small bowl, combine oats, brown sugar, flour, and cinnamon. Cut in margarine until mixture is crumbly. Add walnuts; set aside. In a mixing bowl, combine cake mix, sour cream, banana, and egg. Mix until moistened. Spread half the batter evenly into prepared dish dish. Sprinkle with half the crumb topping. Spread remaining batter over top. Sprinkle top with remaining crumb mixture. Bake for 30 minutes, or until toothpick inserted in center comes out clean.